How Waves & Rain Contribute to Cliff Erosion

Pounding waves at the base of cliffs and the top down action of heavy rain has been the subject of a 3 year investigation to determine how each affects coastal erosion.

This research from California helps with our understanding of the processes involved in coastal erosion and in turn helps to guide how we can protect vulnerable sites where possible.

“It’s something that I’ve been trying to quantify for a long time, which is exciting,” said coastal geomorphologist Adam Young, of the Scripps Institution of Oceanography at UC San Diego .

“We’ve always known that waves were an important part of the cliff erosion process, but we haven’t been able to separate the influence of waves and rain before.

“Anytime a study involves sensors in the coastal zone, it’s a challenge.

His team at Scripps Oceanography’s Center for Climate Change Impacts and Adaptation buries sensors in the sand that measure wave energy. Big surf and erosion can shift the sensors and prevent scientists from collecting reliable measurements.

Adam Young and team deploying erosion monitoring instruments at Torrey Pines State Beach credit Erik Jepsen/UC San Diego

The key to their success, according to Young, was to visit and measure the cliffs every week for three years — an effort that was among the most detailed ever undertaken for studying coastal cliffs. These records along the 1.5 mile-long stretch in Del Mar allowed Young’s team to untangle the effects of rainfall and groundwater runoff from wave impacts.

Adam Young explained:

“We can now better predict how much erosion will occur during a particular storm using the wave and rainfall erosion relationship that we’ve identified here.”

Young’s group combined measurements from the sensors buried in the sand with computer models of wave energy, as well as with three-dimensional maps of the beach and cliffs collected using a LiDAR device – a laser mapping tool – that was mounted onto trucks driven along the beach. The team also analyzed rainfall data from a local Del Mar weather station.

A coastal erosion cliff survey in Del Mar performed by Adam Young and Lucian Parry via a truck-mounted LIDAR system credit Photo credit: Erik Jepsen/ UC San Diego

Because rainfall and associated elevated groundwater levels trigger larger landslides, cliff erosion generally appears to be more correlated with rain. Teasing out the wave-driven cliff erosion is a more subtle and difficult process, but important because the wave-driven erosion weakens the cliff base and sets the stage for those rain-driven landslides.

Understanding the way that cliffs and waves behave together will help improve short-term models that forecast cliff retreat, but the researchers will need more information to predict how future rainfall and waves will drive cliff erosion in the long term.

Young and his group plan to continue to collect data in Del Mar, and are developing a website to make the information about the conditions leading to coastal landslides readily available.

The study appears in the journal Geomorphology and was funded by California State Parks. 

Marwick Head from Birsay Brough car park, B Bell

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